1. Field of the Invention
The present invention relates to circuits for processing video signals and more particularly to circuits for improving the television picture quality of a standard broadcast television signals such as NTSC signals.
2. Description of the Prior Art
Efforts are constantly being made to improve television picture quality. Many different picture "enhancers" have been developed, with varying degrees of success. Most enhancers have various problems associated with them, primarily in terms of distortion of the picture.
Television systems having increased resolution are commonly referred to as high definition television (HDTV). Such systems generally fall into three categories. The first category includes NTSC-type systems with evolutionary improvements which give the appearance of higher resolution. For example, various interference effects can be greatly reduced by the use of advanced "comb" filters and by using a digital framestore to eliminate the standard "interlaced" scanning method. A second category retains the 525-line and 4:3 aspect ratio of NSTC TV but employs non-NSTC encoding and requires a wider bandwidth. A third category is completely incompatible with NTSC-type systems and typically includes at least double the number of scan lines as are used in NTSC systems. The major problems associated with the second and third categories of HDTV are partial or complete incompatibility with the standard NTSC system, with resultant high costs associated with changing over to a new system. Therefore, it is desirable to provide some means of increasing picture resolution while maintaining the use of the standard NTSC format.
Circuits for improving television picture quality are disclosed in U.S. Pat. No. 3,859,544 to Nero, U.S. Pat. No. 3,935,384 to Jirka, U.S. Pat. No. 3,938,181 to Avins, U.S. Pat. No. 4,030,121 to Faroudja, U.S. Pat. No. 4,074,308 to Gibson, U.S. Pat. No. 4,268,864 to Green, and U.S. Pat. No. 4,402,006 to Karlock. In the Nero patent, a delay circuit is disclosed for delaying luminance information with respect to chroma information. In addition, a "crispness" circuit is discussed in which preshoot and overshoot in the luminance circuit is provided. In Kurka, a circuit for modulating scan velocity is disclosed to improve picture quality. Avins discloses a circuit in which the bandwidth of the luminance signal is controlled as a function of the amplitude of chrominance information. Faroudja discloses a video crispener in which video signals are differentiated twice and added to the original signal in order to give the appearance of increased bandwidth. In Gibson, a delay line is employed in which a plurality of delayed signals are combined in a controlled fashion in order to accentuate high frequency portions of a video luminance signal. Green discloses an enhancement system in which a fraction of a composite detail signal representative of amplitude variations of the video signal is subjected to processing and added to the delayed video signal to provide a picture with reduced noise while maintaining detail. Karlock discloses an enhancer in which picture detail is enhanced by suppressing large transitions in a video signal and adding the suppressed signal to a main video signal.